Article pubs.acs.org/JPCC
Nanorod Formation by Photochemical Metal Deposition in Nanoporous Aluminum Oxide Templates Julia Katzmann* and Thomas Har̈ tling Fraunhofer Institute for Nondestructive Testing, Dresden Branch, Maria-Reiche-Str. 2, 01109 Dresden, Germany ABSTRACT: We report on a photochemical approach for the deposition of gold into the nanosized pores of an anodized aluminum oxide (AAO) template and thereby form elongated nanoparticles. The photochemical reduction of a gold salt complex is triggered by the presence of seed particles applied on the pore openings, which results in a targeted deposition of the optically activated gold species from solution. The Au nanorods generated inside the AAO template are analyzed by optical extinction spectroscopy and electron microscopy. These investigations allow conclusions concerning the distribution of the particle aspect ratio and at the same time provide a deeper understanding of the diffusion-driven deposition process inside the pores. On the basis of these results, we discuss several deposition parameters and highlight advantages and difficulties of the technique.
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INTRODUCTION Metal nanoparticles carry great potential for optical sensing applications due to their capability to locally enhance electromagnetic fields via plasmonic resonances. These resonances can be used to confine and enhance light-matter interactions on the nanometer scale.1,2 The central challenge for exploiting these optical properties in economically realistic sensing schemes is the cost-effective fabrication of nanoparticles and particle-based nanostructures. Expensive fabrication approaches such as elaborate lithography methods3,4 or complex chemical synthesis routes relying on precise process parameter control5,6 are suitable for laboratory-based investigations but hardly present options for the preparation of large lot sizes. Bearing this issue in mind, many investigations of cheap and simple fabrication routes for metal nanoparticles were conducted and reported in the past decade. In these experiments, the case of elongated nanoparticles has proven to be particularly challenging, as several rather simple lithography approaches such as nanosphere lithography7 or hole-mask-lithography8 meet their limits here. For this reason, template-based methods were developed, with the exploitation of nanoporous anodized aluminum oxide (AAO) being one of the most prominent ones. The self-organized pores of AAO matrices can be electrochemically filled with metal9,10 resulting in elongated nanoparticles, which reflect the dimensions of the template pores. To extend and further simplify this approach, we report here on a photochemical method for filling the AAO pores with metal. This approach proves advantageous as there is no need of electrically contacting the matrix. Such photochemical experiments were initially reported by Zhao et al.11 who deposited gold into the pores of AAO to synthesize Au nanorods. However, in these works empty AAO templates were © 2012 American Chemical Society
entirely immersed in an irradiated gold salt solution. Hence, nanoparticles formed at arbitrary locations inside the pores. In the present study, we define the anchor point for photochemical gold deposition by nanoscale metal seeds. Such seeddriven gold deposition was exploited previously to manipulate 2D metal nanostructures on flat surfaces12−14 and is extended here to the 3D case. We demonstrate that the implementation of catalytically active seeds in AAO matrices enables the fabrication of ordered and aligned metal nanorods. At the same time, the technique allows a more detailed understanding of the deposition mechanism inside the AAO pores. In the case of gold, the deposition relies on the optically induced formation of activated Au ions in a solution, which are further reduced to elemental metal upon contact with the catalytically active seed surface. Hence, the diffusion of gold species in the solution is a crucial parameter for the rate and spatial distribution of the deposition, which we investigate here via optical extinction measurements and scanning electron microscopy (SEM) imaging. This manuscript is organized as follows: We start with a description of the experimental procedure for the preparation of AAO matrices and the photochemical gold deposition. We then turn to the results of the optical investigations and SEM imaging of the AAO−gold composite. Finally, we analyze several deposition parameters (with an emphasis on diffusion) and highlight advantages and difficulties of the technique. Received: April 23, 2012 Revised: September 18, 2012 Published: October 18, 2012 23671
dx.doi.org/10.1021/jp303896a | J. Phys. Chem. C 2012, 116, 23671−23675
The Journal of Physical Chemistry C
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Article
Photochemical Metal Deposition. The applied gold salt solution was a 200 mM solution of HAuCl4 (abcr, 99.9%) in ethylene glycol. We found that this concentration value represents a threshold for deposition experiments with the protocol described here, that is lower concentrations led to unsufficient deposition (data not shown). A quantity of 5 μL of the solution was pipetted onto the uncoated template side (sample area 25 mm2). Capillary forces drive the solution into the open pores of the hydrophilic template and bring it into contact with the metal seed layer (part b of Figure 1). Then, a homogeneous irradiation of the entire droplet with white light (75 W xenon arc lamp, LOT-Oriel) was immediately started. Upon irradiation, the photochemical reduction of HAuCl4 proceeds as follows:16
EXPERIMENTAL SECTION Preparation of the Nanoporous AAO Template. As a template for gold nanorod synthesis, porous anodic aluminum oxide (AAO) was produced following the two-step procedure described by Masuda and Satoh.15 First, a sheet of high purity aluminum (abcr, 99.9995%) was prepared by cleaning, tempering, and electropolishing. Afterward, the aluminum was anodically oxidized twice in an oxalic acid electrolyte at a temperature of 5 °C and a constant voltage of 40 V. The oxide formed in the first anodization step was dissolved in a chromic acid bath. After the second anodization, the remaining aluminum was removed by a copper chloride solution and the remaining barrier oxide at the pore bottoms was dissolved by 5 wt % phosphoric acid. This procedure resulted in aluminum oxide templates with 15 μm thickness and nearly hexagonally ordered open pores. The interpore distance and pore diameter amounted to 100 and 40 nm respectively resulting in a pore aspect ratio of ca. 375. To provide metal seeds to which the optically activated gold ions can attach, a small amount of gold (
